In a node in a wireless communication system, there is sometimes a need for using a node such as a radio base station (RBS) with a main unit (MU) that has fewer base-band branches than the number of radio branches in a radio remote unit (RRU).
One scenario is when antennas and RRU:s deployed for one system should be re-used for another system. This system may be deployed with RBS:s that have MU:s with fewer base-band chains than the number of branches in the deployed RRU:s.
Another scenario is when a system is first deployed using MU:s with relatively few base-band branches, but is expected to be migrated to MU:s with more base-band branches as the system evolves. In order not to be forced to replace already deployed antennas and RRU:s, it may be desirable to use RRU:s with many branches already at the beginning, and later be able to upgrade the system. It is then sufficient to only upgrade the MU:s to more branches along the migration path.
A simple solution is to connect each base band chain to one radio branch, leaving the excessive radio branches unused. Another solution is to connect one base band chain to two or more adjacent radio chains. If these radio chains are connected to antenna elements with the same polarization, the resulting beam will have a narrower beam-width than the individual physical antenna element.
When power amplifiers are used, the solutions described above do not fully utilize the power amplifiers or preserve the beam-width of the antenna element patterns. In order to maximize the total output power, all power amplifiers should be fully utilized. In order to retain the same cell coverage, the resulting beams should have the same beam-width as the individual antenna elements
There is thus a desire to take care of the total capacity of a node where there is a connection between a first number of base-band branches and a second number of radio branches or antenna ports, where the second number is higher than the first number.